Forming isolation regions with low parasitic capacitance and reduced damage

A method comprising:

• forming a plurality of semiconductor regions that are elongated in a top view of the plurality of semiconductor regions, wherein the plurality of semiconductor regions are parallel to each other;
• forming a plurality of source/drain regions starting from the plurality of semiconductor regions;
• forming a first gate stack and a second gate stack that are elongated in the top view and parallel to each other, wherein the first gate stack and the second gate stack cross over the plurality of semiconductor regions;
• and
• replacing portions of the first gate stack and the second gate stack to form a first dielectric isolation region and a second dielectric isolation region, respectively, wherein the first dielectric isolation region is wider than the second dielectric isolation region.

A method comprising:

• etching a first gate stack and a second gate stack to form a first opening and a second opening, respectively, wherein the first opening is wider than the second opening;
• etching a first protruding fin and a second protruding fin that are underlying and exposed to the first opening and the second opening;
• etching a first semiconductor strip and a second semiconductor strip that are underlying and exposed to the first opening and the second opening;
• and
• filling the first opening and the second opening to form a first fin isolation region and a second fin isolation region, respectively.